The present invention relates to a method of controlling automatic transmissions during traction control.
An automatic transmission is constructed to determine a power transmission route or gear ratio of a transmission gear through selective hydraulic operation or engagement of friction elements such as clutch and brake, and carry out shifting to another gear ratio through switching of friction elements actuated. Determination whether or not shifting should be made is carried out in accordance with predetermined shift lines defined by the opening of a throttle valve of an engine and the vehicular speed. Specifically, determination of shifting to another gear ratio is normally carried out when the operating state given by the throttle valve opening and the vehicular speed crosses the corresponding shift line.
Referring to FIGS. 3A-3C, the above predetermined shift lines are generally established as indicated, e.g. by fully-drawn lines in FIGS. 3A-3C in connection with a dry road on which a motor vehicle cruises the most frequently. Experience reveals that the following problem arises when traction control is carried out to reduce engine output upon occurrence of a skid of a driving wheel on a low-friction road.
In the high throttle valve opening area, there is an excessive tendency to select a low gear ratio, increasing the engine speed, resulting in enlarged rotational inertia energy of the engine upon shifting. Since the enlarged rotational inertia energy is apt to cause a skid of a driving wheel upon shifting, traction control often comes into action, having a tendency to degraded operability of the motor vehicle. On the other hand, in the low throttle valve opening area, there is an excessive tendency to select a high gear ratio, decreasing the engine speed, which can be lower than a lower limit value thereof which allows traction control. Thus, impossible traction control can occur, resulting in possible degradation of the operability of the motor vehicle.
As for conventional shift line control during traction control, JP-A 64-32934 proposes an art of changing the entirety of a downshift line in the direction of low vehicular speed during traction control.
With such art of changing the entirety of a downshift line during traction control, however, if a shift line is changed to solve the inconvenience in the low throttle valve opening area, the inconvenience in the high throttle valve opening area becomes more striking than that in the conventional art. On the other hand, if a shift line is changed to solve the inconvenience in the high throttle valve opening area, the inconvenience in the low throttle valve opening area becomes more striking than that in the conventional art.
It is, therefore, an object of the present invention to provide a method of controlling automatic transmissions during traction control without any degradation of the operability of the motor vehicle.